Its an RPA file, not a zip (etc) file, so it doesn't need unzipping. You need to find it in your downloads folder and move it to the right game folder. That's it.
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Yeah, the game uses rpa files to run, and the instructions say to unzip then move the rpa file, so I think the instruction to unzip is based on an assumption that itch is providing a compressed file when it's not.
As for starting over, that depends on where you're at, but as a rule when I mod something, I start over to ensure everything works.
Based on other q&a threads here, there's no indicator that it's installed correctly. You have to find Amy for the first time and her path with either be available or it won't.